When consumers buy a service, they enter into a contract with the business providing that service. Consumers can expect the service under the contract to be delivered with reasonable care and skill" regardless of whether the contract make direct reference to the standard or not.
The law states that "reasonable care and skill" is an implied term into all service contracts, which means that where it is not expressly set out in the contract, it still applies. To do otherwise would result in negligence on the part of the business (negligence can be used as shorthand for the lack of reasonable care and skill). However, consumers may not be aware that this implied term is present in their favour.
In some circumstances, businesses may have a clause that (attempts to) exclude or limit their liability for negligence in a contract with a consumer, ie limit the extent of "reasonable care and skill" they must show when supplying the service or at the very least limit their liability arising out of the failure to display reasonable care and skill.
This can only be done where the supplier and the customer expressly agree to the limitation or it is consistent with the way both parties have acted. It is sufficient for the supplier to include such limitations on their standard terms and conditions where the consumer has implicitly or explicitly agreed to them. The exclusions must be considered to be "reasonable" or they will not be enforceable.